The invention relates to the separation of a liquid and/or gaseous phase from a solid phase and particularly to filters for preventing the solid particles of a media bed from entering the flow apertures in a conduit which may distribute fluid to the bed or collect fluid from the bed. For example, in a certain process for treating a municipal water supply to remove color, odor and to oxidize biofouling aqents, the raw water is caused to flow downwardly through a filter media bed of activated charcoal and to be collected at the bottom of the bed. To insure the most complete treatment of the water, compressed air is injected into the bed from tiny apertures in an array of pipes located at the bottom of the bed. The air serves to improve contact between the charcoal particles and water and thus reduces the residence time required. Whenever the air flow in the air injection pipes is shut off, the media particles, which are of a lesser dimension than the pipe apertures, would normally tend to enter the pipe. To prevent this, it has been common practice for the apertures to be covered with one or more layers of a metal or plastic filtering cloth applied directly over the openings. The cloth restricts the flow to a lesser or greater extent, depending upon the number of layers used. A single layer allows more flow but is quite subject to damage by being crushed and broken by the weight of the particles. Multiple layers are stronger but would increase the back pressure, would be relatively easily clogged, would increase the filtration velocity and thus possibly break down the media, and would increase corrosion and abrasion of the pipe. The aforementioned problems have resulted in the need to frequently close down the operation, remove the filter media, clean the pipes, and replace the filter cloths. Premature shutdown is very undesirable since a typical bed might be 70 m long, 35 m wide and 1 m in depth. Besides water treatment processes using activated charcoal, similar problems arise in other media bed processes and with other media such as resin, sand or alumina. For example, in a nuclear power plant condenser, evaporation of the cooling water increases the concentration of the minerals in the water. The minerals can be removed in a filter bed containing fine particles of resins.